Structures and methods of manufacture for gas diffusion electrodes and electrode components

ABSTRACT

Gas Diffusion Electrodes (GDEs) play a pivotal role in clean energy production as well as in electrochemical processes and sensors. These gas-consuming electrodes are typically designed for liquid electrolyte systems such as phosphoric acid and alkaline fuel cells, and are commercially manufactured by hand or in a batch process. However, GDEs using the new electrolytes such as conductive polymer membranes demand improved electrode structures. 
     This invention pertains to GDEs and gas diffusion media with new structures for systems using membrane electrode assemblies (MEAs), and automated methods of manufacture that lend themselves to continuous mass production. Unexpected improvements in gas and vapor transport through the electrode are realized by incorporating a new dispersion process in the construction, reformulating the applied mix with solution additives, and creating a novel coating structure on a conductive web. Furthermore, combining these changes with a judicious choice in coating methodology allows one to produce these materials in a continuous automated fashion.

PRIOR APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/585,124 filed Jun. 1, 2000, now U.S. Pat. No. 6,368,476, which is acontinuation of U.S. patent application Ser. No. 09/184,089 filed Oct.30, 1998, now U.S. Pat. No. 6,103,077 which is based on provisionalapplication Ser. No. 60/079,342 filed Jan. 2, 1998.

A gas diffusion electrode (GDE) consumes or is depolarized by a gas feedwhile allowing direct electronic transfer between the solid and gasphase. Together with the electrolyte, the GDE provides a path for ionictransfer, which is just as critical. GDEs are typically constructed froma conductive support, such as a metal mesh, carbon cloth, or carbonpaper. This support is often called a web. The web is coated withhydrophobic wet-proofing layers, and finally, a catalytic layer isapplied most commonly to one face. While the catalytic layer can consistof very fine particles of a precious metal mixed with a binder, manyemploy the methods similar to that in Petrow, et al., U.S. Pat. No.4,082,699. This patent teaches the use of using finely divided carbonparticles such as carbon black as the substrate for small (tens ofangstroms) particles of the nobel metal. Thus called a “supported”catalyst, this methodology has shown superior performance andutilization of the catalyst in electrochemical applications. However,the application of this supported catalyst as well as wet proofinglayers to the web engages the need for a well-dispersed mix.

Often, GDEs are cited as key components in Fuel Cells. Here, the anodeis typically depolarized with hydrogen while the cathode is depolarizedwith oxygen or air. The resulting products are energy in the form ofelectricity, some heat, and water. Examples of acid or alkaline fuelcells are well known. However, some have also realized that theenergy-producing quality of a fuel cell can be adapted to industrialelectrochemical processes and thus save energy and hence reduceoperating costs.

GDEs also may allow the creation of a commodity directly from a gaseousfeedstock. For example, Foller, et al. (The Fifth International Forum onElectrolysis in the Chemical Industry, Nov. 10-14, 1991, FortLauderdale, Fla., Sponsored by the Electrosynthesis Co., Inc.) describethe use of a GDE to create a 5 wt. % hydrogen peroxide in caustic. Inthis case, oxygen is the feedstock and a specific carbon black (withoutnoble metals) is the feedstock and a specific carbon black (withoutnoble metals) is the catalyst. A typical chlor-alkali cell uses twosolid electrodes to produce sodium hydroxide and chlorine. In this case,both the anode and cathode expend energy to evolve gas, and specialmeasures are taken to keep the resulting hydrogen away from the chlorinedue to a potentially explosive mixture. The typical chlor-alkali cathodecan be replaced with an oxygen-depolarized cathode, as has been shown byMiles et al. in U.S. Pat. No. 4,578,159 and others. A cell run in such amanner saves approximately one volt, and the hydrogen/chlorine problemis eliminated. Aqueous hydrochloric acid is an abundant chemicalby-product. One can recover the high-value chlorine by oxidizingsolutions of HCl, and thus recycle the chlorine as a feedstock to thechemical plant.

Electrolysis becomes extremely attractive when the standardhydrogen-evolving cathode is substituted with an oxygen-consuming gasdiffusion electrode due to the significant drop in energy consumption.The ability of the gas diffusion electrode to operate successfully inthis and the preceding examples is acutely dependent on the structure ofthe gas diffusion electrode: for in all these cases, the electrodeserves as a zone for liquid-gas-solid contact, as a current distributor,and most importantly, as a liquid barrier. The use of solid polymerelectrolytes has greatly expanded the field of electrochemistry. Assummarized above, electrochemical processes depend on the transfer ofionic and electronic charge through the use of an anode, cathode, and anionic liquid electrolyte. However, with the advent of the solid polymerelectrolyte fuel cell, the traditional liquid phase has been replacedwith a membrane composed of a polymer electrolyte that transfers ioniccharge under typical electrolytic conditions. One can deposit a catalystlayer directly on the membrane, or attach a gas diffusion electrode toone or both faces of the conducting membrane. Such an assembly can becalled a membrane electrode assembly (MEA), or for fuel cellapplications, a PEMFC (proton exchange membrane fuel cell).

These solid polymer electrolytes are often composed of ion-conductingmembranes that are commercially available. For example, in addition tothe previously mentioned Nafion (a cation exchange membrane), AsahiChemical and Asahi Glass make perfluorinated cation exchange membraneswhereby the ion exchange group(s) are carboxylic acid/sulfonic acid ofcarboxylic acid. These companies produce cation exchange membranes withonly the immobilized sulfonic acid group as well. Non-perfluorinated ionexchange membranes are available through Raipore (Hauppauge, N.Y.) andother distributors such as The Electrosynthesis Co., Inc. (Lancaster,N.Y.). Anion exchange membranes typically employ a quaternary amine on apolymeric support and are commercially available as well.

Nafion is typically employed in some fuel cells. For the hydrogen/air(O₂) fuel cell, hydrogen and oxygen are fed directly to the anode andcathode respectively, and electricity is generated. For these “gasbreathing” electrodes to perform, the gas diffusion electrode structuremust be highly porous to allow three phase contact between the solidelectrode, the gaseous reactant, and the liquid or near liquidelectrolyte. In addition to providing a zone for three-phase contact,the gas diffusion electrode structure aids in making electrical contactto the catalyst, enhances transport of reactant gasses into the zone,and provides for facile transport of product away from the zone (e.g.water vapor).

In addition to a gaseous hydrogen fuel and gaseous air (O₂) oxidant,others employ a mixed phase system such as the methanol/air(O₂) fuelcell. Here, liquid methanol is oxidized at the anode while oxygen isreduced at the cathode. Another utilization for ion-conducting membranesand gas diffusion electrodes includes the electrochemical generation ofpure gasses (for example see Fujita et al. in Journal of AppliedElectrochemistry. vol. 16, page 935, (1986), electro-organic systhesis[for example see Fedkiw et al. in Journal of the ElectrochemicalSociety, vol. 137, no. 5, page 1451 (1990)], or as transducers in gassensors [for example see Mayo et al. in Analytical Chimica Acta, vol.310, page 139, (1995)].

Typically, these electrode/ion-conducting membrane systems areconstructed by forcing the electrode against the ion conductingmembrane. U.S. Pat. No. 4,272,353, No. 3,134,697; and No. 4,364,813 alldisclose mechanical methods of holding electrodes against the conductingmembrane. However, the effectiveness of a mechanical method forintimately contacting the electrode to the polymer membrane electrolytemay be limited since the conducting membrane can frequently changedimensions with alterations in hydration and temperature. Swelling orshrinking can alter the degree of mechanical contact.

Thus, an alternative method of contacting the electrodes with thepolymer membrane electrolyte involves direct deposition of a thinelectrode onto one or both sides of the conducting polymer substrate.Nagel et al. in U.S. Pat. No. 4,326,930 disclose a method forelectrochemically depositing platinum onto Nafion. Others have employedchemical methods whereby a metal salt is reduced within the polymermembrane [for example see Fedkiw et al. in Journal of theElectrochemical Society, vol. 139, no. 1, page 15 (1192)].

In both the chemical and electrochemical methods, one essentiallyprecipitates the metal onto the ion conducting membrane. Thisprecipitation can be difficult to control due to the nature of theion-conducting polymer membrane, the form of the metal salt, and thespecific method employed to precipitate the metal. As the goal of athin, porous, and uniform metal layer is often not met viaprecipitation, practitioners have turned to other deposition methods.For example, ion beam assisted deposition techniques are disclosed inco-pending provisional patent application by Allen et al. (Ser. No.60/035,999); a method for coating the membrane with an ink composed ofthe supported catalyst and solvent is disclosed by Wilson and Gottesfeldin the Journal of the Electrochemical Society, volume 139, page L28,1992; and a method of using a decal to deposit a thin layer of catalystonto the ion-conducting membrane is summarized by Wilson et al. inElectrochimica Acta, volume 40, page 355, 1995. Thus, these approachesdiffer from the previous strategy by the catalyst layer being depositedonto the ion conducting membrane, and a gas diffusion structure issubsequently placed against this catalyst layer.

Regardless of whether the catalyst is fixed to the membrane, or coatedonto an uncatalyzed gas diffusion electrode and then bonded to themembrane via mechenical, and/or thermal means, the structure andcomposition of the component contacting the catalyst contributes to theoverall MEA performance. This component is variously called a“diffuser”, an electrode “backing”, “gas diffusion media”, a “gasdiffusion layer”, or an “uncatalyzed gas diffusion electrode” and canpredominate MEA performance during operation at high current density. Wewill use the term diffuser to encompass all these synomyms. A diffuseris a material that: 1) provides electrical contact between the catalystand the electrochemical cell current collector, 2) distributes andfacilitates efficient transport of feed gas or gasses to the electrode,and 3) becomes a conduit for rapid transport of product(s) from theelectrode. Thus the electrode is the catalytic layer or zonecharacterized by a three-phase interface of solid, liquid, and gaswhereas the diffuser is a two-phase structure for gaseous (or liquid)transport and electrical contact.

There are a few commercial providers for diffusers. Gore Associates(Elkton, Md.) offer Carbel™, a conductive, microporous polymer. E-TEK,Inc. (Natick, Mass.) offers uncatalyzed versions of the gas diffusionelectrodes found in their catalog. Of these, the uncatalyzed ELAT™ islisted as the best material for MEA applications. The gas diffusionelectrode structure designed for providing a three-phase zone, currentcontact, and a liquid barrier is being adopted for MEA applications.

Typical ELAT construction is detailed in U.S. Pat. No. 4,293,396 byAllen et al. Here, a carbon cloth serves as the web. Carbon black isprepared for application to the carbon web by using techniques listed inU.S. Pat. No. 4,166,143 whereby solutions of Vulcan XC-72 or Shawiniganacetylene black (SAB) are mixed with water, ultrasonically dispersedwith a sonic horn, mixed with Teflon® (TFE), and filtered. Layers of SABmix serve as the wetproofing layer on each side of the web. Finally,layers of (catalyzed) Vulcan mix are coated onto one side of theassembly. Although the importance of mix penetration into the web isdiscussed, the actual coating method is not disclosed. The reportedproducts were of limited lot size, so may have been individuallyprepared. After the final coat, the assembly may be sintered in air at atemperature sufficient to cause the Teflon to flow, typically 300-350°C. This double sided structure was designed with the intent to create anelectrode that both achieves good gas distribution and contact with thecatalyst while providing a hydrophobic barrier to prevent electrolytetransport completely through the electrode. Regardless, no informationis relayed as to how this structure could be produced with economicalmeans.

Similarly, a typical ink application is described by Ralph et. al. inthe Journal of the Electrochemical Society, Vol. 144, page 3845, 1997and references therein. Here, the goal is to minimize platinum usage. Agas diffusion electrode is constructed by using silk screen technologyto coat a carbon paper web. The ink is comprised of catalyzed carbonblack and binders including Teflon. The authors claim a resultingelectrode structure comparable to that described by wilson andGottesfeld or Wilson et al. cited previously above. If GDEs and ionconducting membranes are to be used in large volume, commercialprocesses such as power generation in electric vehicles, then one mustmeet a significant reduction in component cost. Thus, while the authorsendorse the need for inexpensive manufacturing processes, they describea batch coating design, which inherently limits product throughput.

In both the ink and mix preparation methods, it is generally acceptedthat the ultrasonic horn serves an important role in dispersing thecarbon in solvent. Since the carbons are high surface area substances,it is important to prepare a uniform and stable suspension. Carbonblacks do not “wet-out” without a significant input of energy or shearinto the solution. Some also modify the solution with additives as wellto induce high shear. The ultrasonic horn performs this function ofwetting-out by way of high frequency electrical energy directed from astainless steal tip immersed in the solution. The action of the horngenerates pressure waves through the vessel and produces high shearthrough cavitation. Although suitable for limited production runs or R&Dsized samples, there are several limitations to ultrasound. First, sincethe energy is projected from a single source, i.e., the horn, the poweris a function of the distance from the horn, and will diminishsignificantly as one moves away. Second, as the action of the carbonblack on the horn leads to abrasion and accelerated corrosion, theprojected power spectrum emanating from the horn changes in time. Forthese reasons, ultrasound may not be appropriate for production of largequantities of diffusers.

With the rise of PEMFCs as suitable clean power sources, and theparallel increase in the use of MEAs in industrial and sensorapplications, there is a need for a diffuser tailored for thesematerials. The current diffuser technology employs structures that wereoriginally designed for liquid electrolyte systems. In addition, thecurrent routine use of the sonic horn produces carbon black dispersionsfor coating that may be non-uniform and difficult to control forproduction of large batches of diffuser. Furthermore, the currentmanufacturing methodology is limited in its applicability tocontinuously coating a web—a step believed to be crucial in producing aninexpensive product.

OBJECTS OF THE INVENTION

It is an object of this invention to provide improved diffuserstructures with transport properties for MEA type electrodes.

It is a further object of this invention to introduce a method ofmanufacture that is compatible with continuous automation.

It is a still further object of the invention to introduce a dispersionmethodology that provides an unexpected increase in performance fromdiffusers and gas diffusion electrodes fabricated from carbon blackspreparing using this technique.

These and other objects and advantages of the invention will becomeobvious from the following detailed description.

THE INVENTION

The novel gas diffusion electrode of the invention comprises anelectrically conductive web provided on at least one side with awet-proofing layer of a suitable polymer provided with anelectrocatalyst thereon. The electrically conductive web is preferably acarbon cloth web or carbon paper or a metal mesh. The wet-proofing layermay also contain a dispersion of carbon black such as SAB.

The construction of the standard ELAT grew out of many refinements,geared to producing a general-purpose gas diffusion electrode that wouldwork under numerous electrolytic conditions. Lindstrom et al. (U.S. Pat.No. 4,248,682) and the previously cited U.S. Pat. No. 4,293,396 documentthe progress of the ELAT development. The final structure of the ELATelectrode is determined by the underlying support web, the quantity andkind of carbon black coated onto the web, and the quantity of binder(often Teflon) mixed with the carbon black.

Typically, a final layer of liquid Nafion or ionomer is applied to theface or front of the GDE diffuser to aid in making contact to theelectrode (MEA). Such solutions are readily available and come as a5-10% wt ionomer with an equivalent weight of 1100 or less. Typicallevels of Teflon in the Vulcan mix are 5-80% by weight, more preferably30-70% by weight. The total weight of solid varies by electrode type,but ranges from 0.5 to 25 mg/cm². The weight of solids is determined bythe number of coats applied to the web, and obviously, the weightdelivered per pass by the coating device. While any number of theconducting carbon blacks can be employed, for example ShawiniganAcetylene Black, Vulcan XC-72, Black Pearls 2000, or Ketjen Black, ingeneral, the carbon black selected for wet-proofing is hydrophobic whilethe carbon black selected as the catalyst or electrode layer is morehydrophilic. The Nafion ionomer coated on the face can vary from 0.1 to2 mg/cm² is preferred. FIG. 1 is a schematic to delineate these variouslayers that comprise the structure of the ELAT gas diffusion electrode.

We have changed the structure of the ELAT to accomodate the differentreactant and product transport and electronic contact requirements ofMEAs. FIG. 1 also shows a comparison of the standard ELAT structure totwo embodiments of a new gas diffuser structure. In comparing diffusertype “A” of FIG. 1 to the standard ELAT, one notes both a reduction inthe number of coated layers, which translates to less total depositedsolids, and with coating layers being placed on only one side of thecarbon cloth web. The uncoated side of the web is now oriented towardthe gas feedstream while the coated layers are placed against theelectrode i.e, the face of the membrane electrode assembly). As will beshown in the Examples, these reduced layers and single-sided coatingsallow for a reduced number of fabrication steps, and a thinner, moreopen structure amendable to high gas flux rates.

For diffuser type “A”, there are still two or more types of carbon blackemployed in the architecture of the structure. These are selected so asto create a gradient of hydrophobicity throughout the structure, as wellas to provide a layer than can be more easily wetted at the catalystinterface. However, there are applications where a single kind of carbonblack is appropriate, and diffuser type “B” in FIG. 1 illustrates thisalternative structure. For diffuser type “B”, one or more coats ofcarbon black and binder are applied on one side of the web. Thisdiffuser would be oriented as type “A”, that is, the uncoated side istowards the feedgas plenum while the coated side is against theelectrode face of the MEA. Diffuser type “B” is easier to fabricate, andis the least expensive to manufacture.

While much focus has been made on the structure and performance of gasdiffusion electrodes, little contribution has been made in the natureand effect of carbon black preparation methods for gas diffusionelectrodes. While the sonic horn is frequently cited, we show heresurprising enhancements in diffuser and gas diffusion electrodeperformance through other dispersion methods. For example, one preferredmethod introduces a pressurized flowing stream of solvent and carbonblack in a “Y” shaped chamber that divides the flow into two streams,which are recombined downstream using another “Y”. The effect ofsplitting and recombining the stream introduces high shear and pressuredifferences on the solvent and carbon black, and effectively wets outthe particles in a uniform and consistent manner. A commercial device isavailable through such companies as Microfluidics (Newton, Mass.). Othermethodologies use rotor/stator methodology whereby one set of blades isfixed while the other set is spun at high rates around the fixed set.Such action creates high shear on the sample. Rotor/stator operation areoften performed in batch mode. Another device is a mill where a spinningbarrel with plates performs the function of delivering shear energy tothe solution. Kady Company (Scarborough, Me.) provides a range of thesemachines. These and similar devices are called “homogenizers” andperform the vital function of dispersing solids into solvent in auniform and consistent manner. The following Example section describessuch a preparation and reports results for diffusers and gas diffusionelectrodes unanticipated by simple homogenization of the carbon blacksolution.

While the placement and number of carbon black layers can controlstructure, and the method used to disperse the carbon black alsodetermines performance, the technique employed to coat a web with mixdetermines the final structure as well. The previously cited ELATpatents describe a successful coating on the carbon cloth web resultsfrom physically penetrating into the woven structure to encase the fiberbundles with mix. Thus, the coating methodology most appropriate forthis function is slot-die, knife-over-blade, or spraying followed by aknife operation. Slot-die coating is the preferred method as the slotacts as a control mechanism that meters out a fixed amount of mix. Theweight of solids placed on the web is determined by the line speed, pumprate through the slot die, and mix composition (% solids). Furthermore,since the slot-die acts through creating a constant mass of mix betweenthe slot-die head and the moving web, this coating action serves to bothgive some penetration into the cloth and compensate for surfaceroughness inherent in the cloth.

While slot-die has been used to coat various solid and poroussubstrates, using the slot-die to create gas diffusion electrodes anddiffusers is a novel application. Typical widths of a slot-die rangefrom 5-250 mm, but larger dies can be constructed. The gap of the slotdie can be controlled via shims, but a typical range is between 4 and100 mils, and more preferably 15-30 mils. Both the coating of the mixand the size of the drying sections of the coating machine determine theline speed, as the freshly coated web is next run into a heated chamber.Typical line speeds range from 0.1 to 5 m/min. Multiple coats can beapplied by a series of slot-die stations, or re-running a freshly-coatedweb through the machine. Other attachments to a manufacturing line wouldinclude a continuous sintering oven and a slitting machine to cut thefinal product into the desired dimensions.

For mixes consisting of carbon black or catalyzed carbon black andTeflon, a Gravure style coating method can be employed as well. Gravurecoating employs a spinning rod that is dipped in mix at the lower halfand then contacted with the moving web at the other upper segment.Typically the gravure-coating head spins in a direction opposite thedirection of the moving web, allowing some penetration of the mix intothe web. The quantity of the mix applied to the web per pass iscontrolled by the mix rheology, line speed, gravure rotation speed andgravure imprint pattern, and the area of the web contacting the head.Gravure coating works best with low viscosity mixes.

The selection of a coating method as slot-die, gravure,knife-over-plate, or spraying is dependent on the fluid dynamics of themix, mix stability during the coating process, and the electrode and/ordiffuser structure desired on the web. One is not limited to one coatingmethod. Typically, more than one coating station can be applied to themoving web to build up a multi-layer structure if so desired, whereuponthe selection of coating station is dependent on the requirements of themix.

In some cases, the composition of the dispersed carbon black mix ismodified by adding additives such as iso-propyl alcohol (from 0.1 to100%, more commonly between 5 to 30%, and preferably 25%), Fluorinert FC75 or similar, Neoflon Ad-2CR, polyvinyl alcohol, polyox, or similarstabilizers.

In some operations it is preferable to avoid iso-propyl alcohol, forexample due to the constraints and costs of handling organic vapors, anda water-based mix is employed. For this type of mix, one of more of thefollowing stabilizers and thickeners could be employed: Fluorinert FC 75or similar; Neoflon Ad-2CR; polyvinyl alcohol, ethylene glycol,polyethylene glycol alkyl ether; Polyox®; Triton® X100; Tween®; Joncryl61J, Rhoplex AC-61, Acrysol GS (acrylic polymer solutions); andnaphthalene formaldehyde condensate sulfonates.

The electrocatalyst may be any of those conventionally used such asplatinum or a rhodium—rhodium oxide catalyst described in U.S. patentapplication Ser. No. 013,080 filed Jul. 26, 1998. The specific coatingmethod and stabilizer is dependent on the structure of diffuser desired.

In the following examples, there are described several preferredembodiments to illustrate the invention. However, it should beunderstood that the invention is not intended to be limited to thespecific embodiments.

EXAMPLE 1

A standard ELAT is constructed for comparison with diffuser or gasdiffusion electrode structures of type “A” of “B”. A web consisting ofcarbon cloth with a warp-to-fill ratio of unity, with approximately 25to 50 yarns per inch, and a carbon content of 97-99% was selected froman available thickness of 5-50 mils, preferably around 10 mils. Anappropriate weight of SAB or Vulcan XC-72 was dispersed with anultrasonic horn. A solution of fluorinated hydrocarbon, otherwise calledTeflon, was added to the mix to form a 50% wt (solids) component. To theweb, a first mixture of dispersed SAB was coated onto each side, untilcoverage of approximately 3.5-7 mg/cm² was obtained. This layer wasconsidered the wet-proofing layer. The electrode was air dried at roomtemperature in between each coat. To this dried assembly, a second butsimilar mix of dispersed platinum catalyst on Vulcan XC-72 Teflon wascoated on one side. Between one and eight coats were provided to achievethe desired metal loading, typically 0.2 to 52 mg catalyst/“cm²”. Afterthe final application, the coated fabric heated to 340° C. for about 20minutes. As described, this would be a gas diffusion electrode. To makea diffuser, similar steps are performed except uncatalyzed Vulcan XC 72is employed.

EXAMPLE 2

To construct a gas diffusion electrode or diffuser of type “A” structureof the invention, an identical procedure as outlined for Example 1 wasfollowed, except the SAB/Teflon wetproofing layer was applied to oneside of the web at approximately half the total carbon black loading,i.e. 1.5-3 mg/cm². The catalyst coat and final treatment followed thatas detailed above. To make a diffuser, similar steps were performedexcept uncatalyzed Vulcan XC 72 with a loading range of 0.5-1.5 mg/cm²carbon black was employed.

EXAMPLE 3

To construct a gas diffusion electrode or diffuser or type “B” structureof the invention, an identical procedure as outlined for Example 2 wasfollowed. However, only the SAB/Teflon wetproofing layer or platinumcatalyzed Vulcan XC-72 was applied to one side of the web at at totalloading of approximately 0.5-5 mg/cm². Similar drying and heating stepsas Example 1 followed. A diffuser was constructed in an identical mannerexcept either SAB or Vulcan XC-72 without catalyst was employed.

EXAMPLE 4

A type “B” gas diffusion electrode similar to that of Example 3 wasconstructed through an automated coater. For this example, aknife-over-plate coater was used and the coater employed a 255 mmperpendicular stainless steel blade with a 45° C. beveled edge. Theblade was positioned over the cloth with a fixed gap of approximately 10mils. The line speed was 2 meters/min., and mix, prepared as in Example3, was fed at continuous rate to a reservoir in front of the blade.Samples thus prepared were subjected to the same heating and dryingsteps of Example 1.

EXAMPLE 5

Homogenized mixes of carbon black were created through the use ofMicrofluidic's microfluidizer. A suspension of water and appropriateweights of either SAB or Vulcan XC-72 was fed to the machine, which waspneumatically operated. A single chamber configuration was employedusing the 100 micron chamber, although other chamber sizes could be usedas well. After a single pass through the homogenizer, Teflon was addedto the mix in the same proportion as established in Example 1. The mixwas filtered, and coated onto a carbon web as detailed in Example 1 orExample 2.

EXAMPLE 6

To prepare a diffuser similar to Type “A” with the slot die coatingmethodology, a mix similar to that described in EXAMPLE 5 is prepared,except prior to filtering a finite amount of Triton X100 is added to thecarbon black solutions to make up approximately 1% weight Triton X toweight of carbon black. A typical range for this additive is 0-5% basedon weight of carbon black. Furthermore, some dissolved Polyox is addedto the solution in the amount of 10% weight based on carbon black. Thetypical range for this additive is 0-20% based on the weight of carbonblack. Excess solution is removed.

A mix of Shawinigan Acetylene Black (SAB) or Vulcan XC-72 as preparedabove is placed in a pressurized vessel that is connected to the slotdie. A pressure of 10-15 psi is applied to the vessel to deliver mix tothe slot-die head at an appropriate rate. The 250 mm long slot die isoriented to ride on the carbon cloth web, a gap of 18 mils is set forthe slot die. For both the SAB and Vulcan mixes, the web passes thecoating head at 2 m/min. Multiple coats of SAB and Vulcan are applied tothe web until an appropriate weight of carbon is distributed. The coatedcloth is dried prior to each additional coat. The final assembly issintered at 340° C. for 20 minutes prior to testing.

EXAMPLE 7

To prepare a catalyzed gas diffusion electrode similar to Type “A” withthe gravure coating methodology, a mix similar to that described inEXAMPLE 5 is prepared, except now less water is removed and a lessviscous consistency is attained. Also, the Vulcan XC-72 is catalyzedwith a 30% wt of platinum. A carbon cloth web is rolled past a 12.7 mmdiameter, 250 mm long gravure head that is rotating at 100 rpm. Thisgravure head has a 5.3 cell/cm² pattern across the surface to aid inpick-up and distribution of the mix. The web is first coated with SAB atthe rate of 2 m/min. Several coats are applied with air dryingin-between coats. Next several layers of 30% Pt on Vulcan XC-72 areapplied at 1 m/min, with drying in-between coats. The final assembly issintered at 340° C. for 20 minutes prior to testing.

EXAMPLE 8

In order to illustrate various aspects of these new electrode/diffuserstructures, a series of diffusers is prepared and compared to thecommercially available ELAT™ diffuser. Several Type “A” diffusers ofvarying thicknesses are prepared according to the methods described inEXAMPLE 2 except the total weight of solids is increased or decreased toadjust the final assembly's thickness. A type “B” diffuser of SAB wasprepared according to the methods described in EXAMPLE 3. For both theseType “A” and “B” diffusers, the methodology of dispersion of EXAMPLE 5is employed. Table 1 summarizes the key differences among the backings.The thickness of each diffuser is taken with nine measurements acrossthe entire (100 cm²) sample. A representative thickness of each type ofdiffuser is the total average of these nine measurements and number ofbacking samples.

TABLE 1 Diffusers Diffuser No. Type Description Thickness +/− milsSamples B Uniform thin hydrophobic 17.0 +/− 0.6 4 microporous layer AComposite of very thin 15.6 +/− .2  4 hydrophobic and hydrophilicmicroporous layers A Standard composite of 16.2 +/− 0.7 5 thinhydrophobic and hydrophilic microporous layers A Thick composite of 18.8+/− 1.0 4 hydrophobic and hydrophilic microporous layers StandardCoatings on both sides of 19.4 +/− 0.6 6 ELAT web

For porous and/or fibrous gas filtration media, resistance to flow isoften used as a characteristic measure for quality control andperformance. This measurement is standardized and so widely performedthat a commercially produced instrument is employed, called a “Gurley”device. The Gurley number is the time it takes to move a fixed pressurethrough a fixed area of sample, and thus the Gurley number indicates theresistance to gas flow. As the diffuser permeability is an importantparameter for optimized fuel cell operation, the Gurley number is a goodmethod to quantitatively characterize diffusers.

To determine the ‘Gurley’ number of the various diffusers, an apparatusfor measuring resistance to flow was constructed employing twowater-filled “U” tubes, one 80 and the other 40 cm long, a nitrogen flowmeter (0-20 LPM), and a back-pressure valve. Samples of electrodebacking (10×10 cm.) are cut and fitted into a manifold with gaskets (5×5cm. exposed area), such that the uncoated side of the backing isoriented towards the nitrogen inlet. Prior to evaluation of a backingsample, the system's inherent resistance to gas flow is evaluated bymeasuring back-pressure in the U-tubes over a range of flow rates. Thissystem “resistance” is used as a correction in subsequent diffusermeasurements.

In order to first establish an appropriate evaluation range, fivesamples of standard Type “A” diffusers were subjected to a series ofoutput flow rates. These output rates are fixed by progressivelyincreasing the input flow through the output back-pressure valve and/orinput flow rate, and noting the output flow and input and output U-tubepressure. The output flow was varied from 1 to 7 LPM. Pressuredifferentials, corrected for the system resistance, are calculated, andthe output flow rate in LPM is divided by the differential pressure inunits of cm. of H₂O, which is then normalized to the exposed backingarea or 25 cm². The resulting value is the characteristic constant forresistance to flow and is similar to the Gurley number. A plot of outputflow versus the calculated resistance to flow shows that non-uniformflow is realized at the low and high flow rates, and an output ranges of2-4 LPM is best for these measurements on diffuser materials. Based onthis result, an output flow of 3.0 LPM was selected for subsequentcomparative measurements.

An additional effect of the diffusers on PEM Fuel Cell performance isrelated to its role in helping to maintain water balance within thecell. Water balance in the fuel cell entails a delicate balance ofhydrophobic and hydrophilic character within the backing layer. Thisbalance depends critically on operating parameters such as currentdensity of operation (which determines how much water is generated),humidification conditions and the flow rate of gases into the cell.Thus, depending on how the fuel cell is run with regards to currentdensity, hydration, type of ion exchange membrane, and flowcharacteristics to the diffuser the hydrophilicity of the backing isalso crucial as well as the structure of the diffuser. Thus, toillustrate how different diffuser structures effect transport under aconstant set of operating conditions, these same samples were evaluatedin a single cell of a PEM Fuel Cell testing apparatus.

RESULTS

Samples of ELAT or type “A” gas diffusion electrode were subjected tosmall scale tests in an apparatus designed to remove system influencesfrom the experiment. That is, typical operation of a fuel cell orelectrochemical process may be dependent on cell design, assembly, andsystem control parameters. This test used an electrolyte solution andcatalyzed electrode to eliminate contact variance between the typicaldiffuser and MEA. Thus, the catalyst in this system was “wetted” and theresults reflect electrode structural differences when the same catalystand and catalyst loading were employed.

A gas diffusion electrode holder (1 cm dia.) was constructed whereby thecatalyzed face was positioned in a solution containing 0.5MH₂SO₄ whilethe backside was subjected to an open gas plenum of approximtely 20 cc.A potentiostat and three electrode set-up were employed to preciselycontrol the applied potential to the test sample. A sheet (2.5×2.5 cm)of platinum served as the counter electrode. Standard ELAT samplescontaining 30% Pt/C, 1 mg/cm² were prepared using the method ofExample 1. Type “A” electrodes were fabricated according to the steps ofExample 2, whereupon the catalyst and loading was the same as the ELAT.Each electrode was sprayed with Nafion resulting in a coverage of 0.5mg/cm². After mounting in the holder, samples were immersed in the acidsolution, which had been heated to approximately 55° C. The electrodeswere first fed oxygen at a stoichiometric excess (greater than 10 fold)under a very slight pressure −2 mm H₂O) for conditioning as negativespotentials were applied vs. saturated calomel electrode (SCE) to reduceoxygen. After the exposure to oxygen, the cell was disconnected, flushedwith nitrogen, and hydrogen was fed to the electrode under the sameslight pressure and stoichiometric excess. Positive potentials wereapplied and the current was recorded. No accomodation for IR was made inthe measurements, although 0.5 -lohm had been measured through theelectrode holder. Multiple samples from each ELAT or type “A” gasdiffusion electrode were tested and averaged and the reported error barswere one standard deviation.

FIG. 2 is a plot of these tests. The applied potential is listed on theabscissa, while the resulting current due to hydrogen oxidation isdisplayed on the ordinate axis. Since the catalyst and wettingcharacteristics were identical for these structures, one concludes thatthe increase in current for type “A” over the ELAT gas diffusionelectrode was due to the improved structure of the type “A” electrode.

The surprising effect of homogenized carbon is shown in FIGS. 3 and 4.In this test, standard ELATs were prepared as outlined in Example 1 butnow used homogenized SAB and Vulcan XC 72 as described in Example 5. Thethree different gas diffusion electrodes (standard ELAT, ELAT withhomogenized wet proofing layer, and ELAT with both SAB and Vulcan layershomogenized) were tested using the same testing procedure as detailedabove, except both the oxygen and hydrogen curves were recorded. FIG. 3is the oxygen reduction curve. Although there was some scatter in thedata, attributed to uncompensated IR, it can be seen that as the layerswere progressively homogenized, greater reductive (or negative) currentswere generated for a fixed potential. A significant and unexpectedimprovement in ELAT performance was realized through homogenization ofthe carbon blacks. FIG. 4 is a similar plot except the electrode wasused as an anode in ambient hydrogen and a significant and unexpectedimprovement was shown.

The same electrodes were next assembled in a fuel system to confirm theimprovement. For this test, a standard fuel cell test station (Fuel cellTechnologies, Inc. NM) was used to control and humidify feed gasses,provide an electronic load, and record data from a cell 16 cm². Forthese tests, electrodes were mechanically compressed against Nafion 115to form the MEA. An ELAT with homogenized SAB and Vulcan XC 72 layerswas used as a cathode, and the test was performed in air and oxygen.FIG. 5 shows the average results of five replicate standard ELATscompared to a typical homogenized carbon black ELAT when using air asthe oxidant. For these plots, the fixed cell voltage is listed on theordinate, while the recorded current is on the abscissa. FIG. 6 issimilar to FIG. 5 except pure oxygen was the oxidant. In both FIG. 5 andFIG. 6, an improvement was seen for these electrodes in an actualsystem. It is surprising that a simple processing step, i.e.homogenization, produces such an increase in current for a fixed voltage(greater power).

The next example combined the improvements of carbon blackhomogenization with the new structures. A type “A” diffuser wasconstructed according to the homogenization procedure of Example 5.Since this was a diffuser, it was assembled as part of a MEA whereby thecatalyst layer has been deposited directly on the ion conductingmembrane. In this test, comparison was made with a standard ELATdiffuser and the type “A” structure in a fuel cell set up incorporatinga cell 50 cm². The cell was operated with hydrogen and air, and the loadwas systematically varied. FIG. 7 shows data that revealed animprovement unanticipated by either structural or homogenization changesalone. The trace labeled “old” is the standard ELAT diffuser and thetrace labeled “new” is the new diffuser structure with homogenizedcarbons. At the extreme loads, there was a test of a diffuser's abilityto transport oxygen and water vapor, and it shows a remarkable 1oo%improvement in current density for a cell voltage of around 0.4 volts.This example clearly shows a remarkable synergistic effect between thecarbon dispersion and new diffuser structures.

Although the above examples have used fuel cell tests, diffusers andMEAs can be used in industrial electrochemical processes as well. A type“B” gas diffusion electrode composed of platinum catalyzed Vulcan XC-72pressed against Nafion 430 was constructed and this same style ofelectrode was then manufactured through the knife-on-plate methoddescribed in Example 4. The performances of these cathodes operating asoxygen consuming electrodes in a concentrated HCl solution werecompared. FIG. 8 summarizes the current potential curves derived from6.25 cm² samples of the type “B” assemblies. As shown here, theautomated coating process did not introduce any significant changes inthe structure of the electrode, and no difference in current wasobtained.

A type “B” gas diffusion electrode composed by hand as illustrated inEXAMPLE 2 and then compared to a gravure method machine-coated Type “B”gas diffusion electrode as illustrated in EXAMPLE 7, whereby theplatinum content is within 10% of each other. The two electrodes aretested whereby 16 cm² samples of each are used as anodes and cathodesand evaluated PEM Fuel Cell elements in a hydrogen air mix at 70° C.FIG. 9 summarizes the current potential curves derived from thesesamples. As shown here, the automated coating process did not introduceany significant changes in the structure of the electrode, and nodifference in current was obtained.

A series of diffusers are constructed to illustrate the range ofstructures available for gas diffusion electrode or electrode backingapplications. The gas permeation rate through the series was comparedusing the modified Gurley apparatus. A summary of resistance to flowmeasurements for the three types of electrode backing are shown as a barchart in FIG. 10. For the Type “A”, three different thicknesses ofcarbon black were tested. One would anticipate that the resistance toflow should increase as the diffuser thickness increases. This trend isfollowed by the data. The standard double sided ELAT and thick Type “A”diffuser show a greater resistance to flow than the standard Type “A”.It is interesting to note that the thin Type “A” diffuser shows thegreatest relative standard deviation across the average of measurements,indicating that the microporous coating of carbon black may have randompin-holes and thus show lower resistance to flow than the standard Type“A”. These data demonstrate that one can adjust the porosity andtortuosity in the diffuser structure, and that a suitable range ofresistance to flow constants is over the range of 0.06 to 0.005 LPM/cmH₂O/cm², and more preferably from 0.05 to 0.008 LPM/cm H₂O/cm².

While porosity and tortuosity (as measured on a macroscopic scale by theGurley measurements) do contribute to the performance of electrodebackings (as we will show here), other factors such as catalyst layerpermeability and ionic conductivity, backing hydrophobicity and watertransport through the backing may contribute more significantly. Wesubjected this series to evaluation in a PEM Fuel Cell test apparatus.

A collection of polarization curves obtained for cells with differentdiffusers operating an oxygen, and 13.5% O₂ in N₂ are shown in FIG. 11.The results indicate that backings Type “A” thick and the standard ELATexhibit limiting current behavior at ca. 20% lower current densitiesthan for the other cases. This suggests that the thicker backings, withtheir lower gas permeation rates, exhibit more sensitivity to theparticularly dilute portions of the cathode flow stream.

Although the uniformly hydrophobic backing Type “A” exhibits slightlylower performance relative to the thin and standard Type “B”s at thelower current densities, this diffuser has similar performance at highcurrent density. Thus, there is some merit in creating a diffuser ofcompletely uniform hydrophobicity. These results here indicate that awide variety of structures are suitable for PEM Fuel Cells, and that thestructure of the diffuser (or gas diffusion electrode) has to be matchedto the specific operating conditions as well as cell design. All ofthese structures show an improvement over the standard ELAT design underdilute cathode feeds.

These examples demonstrate that new and unexpected advances inperformance are obtained when combining homogenization in thepreparation steps with the new gas diffusion electrode structures. Theseare fabricated into MEAs by either assembling a diffuser with acatalyst-coated membrane, or by incorporating a gas diffusion electrodewith the membrane. The step of homogenization can be used to preparemixes for automated coating, and the new structures are capable of beingproduced in an automated fashion.

Referring now to the drawings:

FIG. 1 is a schematic of new diffuser and gas diffusion electrodestructures where each layer of carbon black represents a coat, althoughthe depicted number of coats does not limit these embodiments.

FIG. 2 is a three-electrode testing of standard ELAT gas diffusionelectrode versus type “A” electrode and both electrodes were composed of30% Pt/C, 1 mg/cm² loading. Electrodes are constructed according toExample 1 and Example 2 specifications and tested in hydrogen at ambientpressure.

FIG. 3 is a three-electrode testing of homogenized carbon in standardELAT gas diffusion electrode. All electrodes were composed of 30% Pt/C,1 mg/cm² loading. Electrodes were constructed according to Example 5,whereupon either the wet proofing SAB or both the SAB and Vulcan layerswere homogenized. Tested in oxygen at ambient pressure.

FIG. 4 is a three-electrode testing of homogenized carbon in standardELAT gas diffusion electrode. All electrodes were composed of 30% Pt/C,1 mg/cm² loading and constructed according to Example 5, whereuponeither the wet proofing SAB or both the SAB and Vulcan layers werehomogenized. Tested in hydrogen at ambient pressure.

FIG. 5 is a fuel cell testing of standard ELAT gas diffusion electrodecompared to standard ELAT constructed according to Example 5, whereuponboth the SAB and Vulcan layers were homogenized. All electrodes werecomposed of 30% Pt/C, 1 mg/cm² loading and MEA were assembled usingNafion 115. The system was operated with hydrated gasses at 70° C., andhydrogen at 3.5 Bar (absolute) and air at 4.0 Bar (absolute). A two-foldstoichiometric excess of oxidant was fed based on the highest currentdensity.

FIG. 6 is a fuel cell testing of standard ELAT gas diffusion electrodecompared to standard ELAT constructed according to Example 5, whereuponboth the SAB and Vulcan layers were homogenized. All electrodes werecomposed of 30% Pt/C, 1 mg/cm² loading and MEA was assembled usingNafion 115. The system was operated with hydrated gasses at 70° C., andhydrogen at 3.5 Bar (absolute) and oxygen at 4.0 Bar (absolute). Atwo-fold stoichiometric excess of oxidant was fed based on the highestcurrent density.

FIG. 7 is a fuel cell testing of standard ELAT diffuser (labeled “Old)compared to type “A” diffuser constructed according to Example 5(labeled “New”), whereupon both the SAB and Vulcan layers werehomogenized. An identical MEA was employed for each test and the systemwas run at two-fold stoichiometric excess of air based on the 1A/cm²while the hydrogen was continuously varied at two times stoichiometrybased on the load requirements.

FIG. 8 is a comparison of type “B” electrodes made by hand and byknife-over-blade automated coating. Both electrodes were composed of 30%Pt/C, 1 mg/cm² and assembled with Nafion 430 to form the MEA. Oxygen wasat a five-fold stoichiometric excess with the highest recorded current,and a back pressure of upwards to 50 cm. H₂O was employed while asolution of 184 g/l HCl was circulated and kept at 55° C.

FIG. 9 is a comparison of Type “A” electrodes made by hand and bygravure machine coating. Both electrodes were composed of 30% Pt/C: thehand made electrode at 1 mg/cm² while the machine electrode atapproximately 0.9 mg/cm², and assembled with Nafion 115 to make a MEA.The cell was operated at 70° C. with hydrated gasses, and hydrogen at3.5 Bar (Absolute) and air at 4.0 Bar (Absolute). A two-foldstoichiometric excess of air based on the highest current density wasemployed.

FIG. 10 is a bar chart comparing the resistance to flow for a series ofType “A”, a type “B”, and a standard E-TEK ELAT described in Table 1.The average thickness for the group of samples is listed as well as onestandard deviation of error in the bar graph.

FIG. 11 is a comparison of current vs. potential for MEAs employing thevarious diffuser structures cited in FIG. 10. Test conditions: 50 cm²cell, Pt loading 0.15-0.2 mg/cm² for anode and cathode whereby metal isdeposited directly on Nafion membrane.

The cell was operated at 80° C. while the anode and cathode backpressure=30 psig; 1.5 stoichiometric anode and cathode flow rate. Neatoxygen and 13.5% oxygen are employed as the oxidants. Hydrogen is thereductant.

What we claim is:
 1. A method of producing a gas diffusion electrodecomprised of an electrically conductive web provided on at least oneside with a layer of a homogenous dispersion of carbon black, comprisinga) preparing a water-based, organic-free dispersion mixture of a carbonblack with a homogenizer, b) adding a binder to the resulting mixture,c) adding at least one dispersion-stabilizing substance to the mixture.d) applying at least one coat of said water-based, organic-freedispersion mixture to an electrically conductive web by using at leastone coating head, e) drying said dispersion mixture on the web, and f)sintering the resulting electrode at 300-400° C.
 2. The method of claim1 wherein said at least one coating head is selected from the groupconsisting of gravure and slot-die coating heads.
 3. The method of claim2 wherein said first carbon black in said first dispersion mixture ishydrophobic.
 4. The method of claim 2 wherein said carbon black in saiddispersion mixture is hydrophilic.
 5. The method of claim 2 wherein saidat least one stabilizing substance is selected from the group consistingof chlorofluorocarbons, polychlorotrifluoroethylene, polyvinyl alcohol,ethylene glycol, polyethylene glycol alkyl ether, polyoxyethylene,on-ionic surfactants, primary alcohol alkoxylates, polyoxyethylenesorbitan mono-oleate, acrylic emulsions, sodium polyacrylate andnaphthalene-formaldehyde condensate sulfonates.
 6. The method of claim 1wherein said first carbon black in said first dispersion mixture ishydrophobic.
 7. The method of claim 6 wherein after said drying of saidwater-based, organic-free dispersion mixture on the web and before saidsintering of said resulting electrode, a coat of a second water-basedorganic-free dispersion mixture containing a second carbon black isapplied and then dried.
 8. The method of claim 7 wherein said secondcarbon black of said second water-based, organic-free dispersion mixtureis hydrophilic.
 9. The method of claim 7 wherein the second hydrophiliccarbon black is a support for at least one member of the groupconsisting of platinum group metals, platinum group metal oxides, alloysand mixtures thereof.
 10. The method of claim 9 wherein said firsthydrophilic carbon black is a support for at least one of the groupsconsisting of platinum groups metals, platinum group metal oxides,alloys and mixtures thereof.
 11. The method of claim 1 wherein saidcarbon black in said dispersion mixture is hydrophilic.
 12. The methodof claim 1 wherein said at least one stabilizing substance is selectedfrom the group consisting of chlorofluorocarbons,polychlorotrifluoroethylene, polyvinyl alcohol, ethylene glycol,polyethylene glycol alkyl ether, polyoxyethylene, on-ionic surfactants,primary alcohol alkoxylates, polyoxyethylene sorbitan mono-oleate,acrylic emulsions, sodium polyacrylate and naphthalene-formaldehydecondensate sulfonates.